As is well known, rolling bearings have been widely adopted for motors that drive 3.5-inch discs. However, fluid dynamic pressure bearings are increasingly being adopted for small motors that drive smaller discs such as 2.5-inch or 1.0-inch discs instead of rolling bearings.
As shown, for example, in FIG. 6, such a fluid dynamic pressure bearing includes a flanged shaft 1 serving as a rotary bearing member, a stepped closed-end sleeve 4 serving as a stationary bearing member, a thrust retainer plate 5 serving as an annular cover member, and lubricating oil injected into a minute gap formed by those members.
The flanged shaft 1 includes a cylinder portion 2 and an annular flange portion 3 that are integrated with each other. A lower part of the cylinder portion 2 of the flanged shaft 1 serves as a radial dynamic pressure bearing cylinder portion, with radial dynamic pressure generating grooves G1 formed on the outer peripheral surface thereof. Further, an upper part of the cylinder portion 2 of the flanged shaft 1 serves as a rotor-mounting cylinder portion, with a rotor-hub-mounting cylinder portion of a small diameter being formed at its distal end.
The annular flange portion 3 of the flanged shaft 1, which functions as a disc-shaped thrust plate, has thrust dynamic pressure generating grooves formed respectively on its upper and lower surfaces.
The stepped closed-end sleeve 4 has a lower cylinder portion of a small diameter and an upper cylinder portion of a larger diameter. An opening with a still larger diameter is formed at the upper end portion of the upper cylinder portion. The thrust retainer plate 5 is fitted airtight into this opening, whereby the opening of the stepped closed-end sleeve 4 is sealed airtight with the thrust retainer plate 5. Formed at the boundary between the small-diameter lower cylinder portion and the large-diameter upper cylinder portion is an annular step portion serving as the bottom portion of the upper cylinder portion.
Between the flanged shaft 1, the stepped closed-end sleeve 4, and the thrust retainer plate 5, there are formed a cylindrical minute gap R1, an annular minute gap R2, a cylindrical minute gap R3, an annular minute gap R4, a cylindrical minute gap R5, and a disc-shaped minute gap R6. Although somewhat exaggerated in FIG. 6, the sizes of the minute gaps R1 through R6 range from ten to several tens μ in the case of small and thin fluid dynamic pressure bearings used in small and thin motors.
It is to be noted that the minute gaps R3 and R6, each functioning as an oil reservoir, are formed wider than the other minute gaps. Lubricating oil is injected into these minute gaps by a vacuum injection method from the annular opening of the cylindrical minute gap R1.
The inner peripheral surface of the thrust retainer plate 5 forms an outwardly tapered surface. Accordingly, when seen in cross section, the cylindrical minute gap R1 formed between the inner peripheral surface of the thrust retainer plate 5 and the upper outer peripheral surface of the cylinder portion 2 forms a tapered gap that is tapered outwardly from the inner to outer portions of the bearing. The resulting capillary action and surface tension form a capillary seal portion S that functions to prevent the lubricating oil from leaking outside of the bearing. Further, the cylindrical minute gap R3 serves as an oil reservoir for thrust dynamic pressure bearing, and the disc-shaped minute gap R6 serves as an oil reservoir for radial dynamic pressure bearing.
As described above, the one-sided-bag-shaped fluid dynamic pressure bearing shown in FIG. 6 includes the flanged shaft 1, the stepped closed-end sleeve 4, the thrust retainer plate 5 that is an annular cover member, and oil for lubrication injected into a minute gap consisting of the plurality of minute gaps R1 to R6 that are communicated with each other and formed between those components. This minute gap defines a one-sided-bag-shaped minute gap, with the opening of the minute gap R1, which opens to the atmosphere, serving as the only opening of this minute gap.
Although it is not easy to inject oil into such a one-sided-bag-shaped fluid dynamic pressure bearing having a one-sided-bag-shaped minute gap, some injection methods have already been developed to this end, such as the vacuum injection methods disclosed in U.S. Pat. No. 5,601,125 (Patent Document 1), U.S. Pat. No. 5,862,841 (Patent Document 2), and U.S. Pat. No. 5,894,868 (Patent Document 3).
As shown in FIG. 5, for example, a conventional oil injecting apparatus employing a vacuum injection method includes an oil container 11 storing an oil 12 to a predetermined level, a cover member 13 having an oil injection through-passage 15 and a suction/exhaust through-passage 17, a bellows 11a that is secured airtight to the back surface of the cover member 13 at one end and to the opening of the oil container 11 at the other end, an injection tube 14 whose distal end projects straight into the oil container 11 and whose other end is connected to the oil injection through-passage 15, a suction/exhaust tube 16 whose distal end projects straight into the oil container 11 and whose other end is connected to one end of the suction/exhaust through-passage 17 of the cover member 13, a suction/exhaust device connected to the other end of the suction/exhaust through-passage 17 of the cover member 13, a moving device that vertically moves the oil container 11 from an exhaust position (FIG. 5 (A)) with the distal end of the injection tube 14 positioned away from the oil level into an injection position (FIG. 5(B)) with the distal end of the injection tube 14 submerged into the oil, and bearing fixing means for fixing the one-sided-bag-shaped fluid dynamic pressure bearing 10, into which the oil 12 is to be injected, onto the cover member 13 with its surface on the oil injection port side being seated on an O-ring 25.
The suction/exhaust device includes a vacuum pump 22, suction/exhaust tubes 18, 20, 21, and valves 23, 24. The moving device includes a stepping motor 27 and an oil container holder 28. Further, the bearing fixing means forms a part of a holding device (not shown) holding the cover member 13.
Oil injection is performed as follows with such a conventional oil injecting apparatus. First, in the exhaust position (FIG. 5(A)) with the distal end of the injection tube 14 positioned away from the oil level inside the oil container 11, a control device (not shown) opens the valve 23 and closes the valve 24, and activates the vacuum pump 22. As this happens, the inside of the oil container 11 is evacuated through the suction/exhaust tube 16, the suction/exhaust through-hole 17, and the suction/exhaust tubes 18 and 20; at the same time, the inside of the one-sided-bag-shaped fluid dynamic pressure bearing 10, which communicates with the inside of the oil container 11 through the injection tube 14 and the oil injection through-hole 15, is also evacuated into a vacuum.
Then, in this state, the control device drives the moving device to raise the oil container 11, whereby the distal end of the injection tube 14 is submerged into the oil 12. Subsequently, in this injection position (FIG. 5(B)), the control device closes the valve 23 and opens the valve 24. As this happens, the inside of the oil container 11 is communicated with the atmosphere through the suction/exhaust tube 16, the suction/exhaust through-hole 17, and the suction/exhaust tubes 18 and 21, and thus turned into the atmospheric pressure. This causes the oil 12 in the oil container 11 to be injected into the one-sided-bag-shaped fluid dynamic pressure bearing 10.
Incidentally, the conventional oil injection apparatus described above is equipped with an extendable part such as a bellows for vertically moving-the oil container. This bellows is formed of a rubber material and hence liable to deform when placed in a high vacuum. Such deformation of the bellows makes it difficult to maintain a high vacuum degree with good accuracy, resulting in poor durability. This leads to a problem in that the bellows must be exchanged frequently in order to maintain this accuracy. In short, the conventional oil injecting apparatus described above is poor in operability, resulting in an increase in maintenance cost.
An object of the present invention is to provide an oil injecting apparatus for injecting oil into a one-sided-bag-shaped fluid dynamic pressure bearing by a vacuum injection method, the apparatus providing good operability and low maintenance cost.